O LDAP não tem nada a ver com o email. É como comparar maçãs e laranjas.
LDAP :
The Lightweight Directory Access Protocol, or LDAP (pronounced /ˈɛl dæp/), is an application protocol for querying and modifying directory services running over TCP/IP.1
A directory is a set of objects with attributes organized in a logical and hierarchical manner. A simple example is the telephone directory, which consists of a list of names (of either persons or organizations) organized alphabetically, with each name having an address and phone number associated with it.
Veja a pergunta do Serverfault "Existe algum servidor Exchange de código aberto?" para algumas respostas realmente boas à sua pergunta real , uma alternativa de código aberto ao Microsoft Exchange.